The electrostatic capacitance sensing type fingerprint image input device is known from, for example, JP Patent Kokai Publication JP-A-8-305832. FIG. 8 is a perspective view showing the configuration of the electrostatic capacitance sensing type fingerprint image entry device which is disclosed in the cited JP Patent Kokai Publication JP-A-8-305832.
Referring now to FIG. 8, this fingerprint entry device comprises a substrate 111 and a multiplicity of electrodes 112 and switching elements 113, which are two-dimensionally arrayed on the substrate 111. The electrodes 112 and switching elements 113 are connected to an electrostatic capacitive sensing circuit 116 and drive circuit 117 via Y leads 114 and X leads 115, respectively.
Now, operation of the prior art fingerprint image entry device which is shown in FIG. 8 will be described. When a finger F is brought closer to the substrate 111, electrostatic capacitances 122 and 123, etc. are formed between the finger F and the electrodes 112. Since the capacitance reflects information on the ridges and valleys of the finger, an image of the fingerprint can be obtained by sensing the capacitance. Sensing of the capacitance 122, 123, etc. between the finger F and the electrodes 112 is conducted as follows.
Firstly, all the parasitic capacitances 121 of the X leads 115 are precharged to a given potential, followed by turning off one of the Y leads 114.
Subsequently, the electrostatic charge is distributed between the capacitance 122 and the capacitance 121 by turning on this Y lead 114. The capacitance is determined by sensing the change in the potential on the X lead 115 at this time.
This operation will be conducted for all Y leads so that the distribution of the electrostatic capacitance between the finger F and each of electrodes 112, that is, a fingerprint image is obtained.